1. Field of the Invention
The present invention relates to a power supply circuit including a regulator capable of being switched ON/OFF.
2. Description of the Related Art
Power supply circuits, each including a regulator capable of being switched ON/OFF, have been heretofore known. Japanese Patent Application Laid-open Publication No. H8-272461 has disclosed a power supply circuit including a standard voltage generator, a regulator, a low-pass filter connected between the standard voltage generator and the regulator.
The regulator in the power supply circuit includes a differential amplifier and a control transistor. To the differential amplifier, a reference voltage and a feedback voltage are inputted. The reference voltage is outputted from the standard voltage generator, and has a voltage value almost equal to that of a standard voltage. The feedback voltage is fed back from the output of the regulator. The control transistor is provided to the output of the differential amplifier. In addition, the low-pass filter is configured of a resistance and a capacitor. The resistance is connected in series between the standard voltage generator and the regulator. The capacitor is connected in parallel to the regulator between the standard voltage generator and the regulator.
In this power supply circuit, once the reference voltage is inputted to the differential amplifier from the standard voltage generator while the feedback voltage is inputted to the differential amplifier from the output of the power supply circuit, the differential amplifier compares the two voltages. On the basis of a result of the comparison, the differential amplifier controls the control transistor. Thus, the power supply circuit keeps its output voltage constant. In addition, the low-pass filter connected between the standard voltage generator and the differential amplifier inhibits a ripple voltage.
In a case where, however, the technique disclosed in Japanese Patent Application Laid-open Publication No. H8-272461 is applied to the power supply circuit capable of switching ON/OFF the differential amplifier in the regulator, the power supply circuit has a problem that a rush current is outputted from the regulator when the regulator is switched from OFF to ON.
Specifically, when the regulator is switched from OFF to ON while the standard voltage is outputted from the standard voltage generator, the reference voltage having the voltage value equal to that of the standard voltage is momentarily inputted to the differential amplifier in the regulator. That is because electrical charges have been already accumulated in the capacitor in the regulator.
On the other hand, when the regulator is switched from OFF to ON, the feedback voltage fed back from the output of the regulator to the differential amplifier is almost zero volts. For this reason, the differential amplifier which compares the reference voltage with the feedback voltage controls the control transistor so that the control transistor can flow a larger current. That is because the difference between the reference voltage and the feedback voltage is large. Accordingly, the rush current flows via the control transistor from the regulator to electronic parts and the like which are connected to the regulator.
This results in a problem that the electronic parts and the like which are connected to the regulator are damaged. Moreover, this invites a problem that the rush current decreases the voltage of a battery which supplies electronic power to the regulator and other equivalent voltages, so that the electronic parts malfunction. Incidentally, for the purpose of preventing a rush current, a technique for including an overcurrent limiter in a regulator has been known. Nevertheless, this technique brings about a problem of complicating the configuration of the regulator.